Fiber Bragg gratings have been successfully integrated as distributed strain sensors in a number of industrial and military structures, such as bridges, highways, dams, spacecraft, and ship hulls. In addition to their diminutive size and weight and immunity from electromagnetic interference, their potential for large scale multiplexing is an advantage over conventional strain gauges. As sensor fabrication technology moves toward low-cost mass production, there is increased motivation for higher multiplexibility and bandwidth. Normally, each Fiber Bragg grating reflects light at the specific wavelength for which it was written. However, a change in the environment of the fiber Bragg grating (e.g. strain in the fiber at the location of the grating, or a change in temperature) results in a shift in the reflection wavelength of the grating. As the shift is proportional to the change in the measurand, the grating may be used as a sensor by measuring the magnitude of the wavelength shift. This requires interrogating the grating, i.e. illuminating it with tunable or broadband optical sources and measuring the wavelength either directly or indirectly. Established interrogation approaches, based on tunable filters (fiber Fabry-Perot or acousto-optic based), tunable lasers, or a charge coupled device spectrometer, all rely on wavelength division multiplexing, and a broadband (or broadly tunable) light source. Although these methods are effective, the number of fiber Bragg grating sensors along a single fiber is limited, since unambiguous detection requires allocating a slice of the source's limited spectral bandwidth for each sensor channel. The spectral width of each channel is proportional to the maximum anticipated strain, which for most applications ranges from 0.1 to 1%. For a typical source whose spectral width is less than about 3% of its center wavelength, this relates to a maximum sensor count per fiber which ranges from approximately 2 to 20 respectively. Therefore, wavelength division multiplexing alone is insufficient for interrogating large sensor arrays (e.g. greater than one hundred) in a single fiber.
A significant increase in capacity can be realized by combining wavelength division multiplexing with time division multiplexing in a hybrid system. Time division multiplexing involves illuminating the gratings with a pulsed light source, then electronically or optically gating the returned pulses in time. In the hybrid system, multiple copies of the fiber Bragg grating subarrays, each containing multiple elements which have different Bragg wavelengths, are combined in a single fiber to form one large array. The replica subarrays are interrogated by time division multiplexing, while intra-array interrogation is achieved via wavelength division multiplexing.
Unfortunately, to achieve uniform illumination of multiple subarrays, the fractional optical power consumed by spectrally overlapping gratings of each subarray must be limited to only a few percent rather than 80-90% which is typically used. This is required to prevent "shadowing" of the distant gratings by the proximal. However, this can be a serious limitation, since such a sacrifice in signal (approximately a factor of 20) can rarely be tolerated due to the low spectral brightness of most sources. With appropriate detection methods a few broadband light sources exist with sufficient spectral brightness for realizing time division multiplexing with low reflectivity fiber Bragg gratings. These include a pulsed 810 nm tapered waveguide amplifier used as an amplified spontaneous emission (ASE) source; an externally modulated, diode pumped, erbium fiber ASE source; and a passively mode-locked erbium fiber laser. The high spectral brightness and unusually broad spectrum of the mode-locked fiber laser make it ideal for wavelength division multiplexing.